User requirements for electrophotographic techniques used in copiers, printers, facsimile receiving devices, and the like are becoming more and more demanding year by year as devices develop further. According to recent trends, it has become essential that high-speed printing of many sheets is possible, and that high image quality can be maintained regardless of the environment because the use environment has become diversified with the expansion of the market.
In order to satisfy the requirements described above, toners having high durability and high stability have been needed more than ever, and various studies have been conducted. For example, as a method for evaluating the chargeability of toner, measurement of a thermally stimulated current has been disclosed (for example, refer to Patent Literature 1). Furthermore, it has been disclosed that, when the thermally stimulated current has a specific value, a toner having a good charging characteristic can be provided (for example, refer to Patent Literature 2 to Patent Literature 5).
In each of Patent Literature 2 and Patent Literature 3, by defining first and second thermally stimulated current values in specific temperature ranges, the existential state of a wax on the toner surface is estimated. This makes it possible to obtain a toner having an excellent charging characteristic, thus realizing high image quality regardless of the environment.
Patent Literature 4 discloses a technique in which a thermally stimulated current in a toner has two or more peaks in a specific range, and on the basis of the relationship between the peak values, a toner having good rise of charging and good retentivity of charge is obtained. In this way, even in a state of being left to stand for a long period of time, it is possible to immediately obtain a sufficient charge amount, which makes it possible to shorten the start-up time.
Patent Literature 5 discloses that it is possible to provide a toner having high durability and high charge stability on the basis of the temperature at which a thermally stimulated current is generated in a toner and hardness of toner.
In each of the patent literatures described above, the charging characteristic is defined on the basis of the peak current value at a certain temperature in the measurement of a thermally stimulated current in the toner. However, in such a definition, the charging characteristic of toner under various environments, such as under a high-temperature, high-humidity environment and under a low-temperature, low-humidity environment, is not defined. Therefore, in the toners described in the patent literatures described above, there is still room for improvement in terms of retention of charge under a high-temperature, high-humidity environment and inhibition of excessive charging under a low-temperature, low-humidity environment.